The present invention relates to an arrangement for controlling braking of a vehicle, and particularly to an arrangement which senses a parameter, e.g. speed of or load on the rear wheels, to adjust brake application on the front wheels.
The invention is applicable to motor vehicles, and particularly to tractor vehicles intended to haul a trailer or a semitrailer by means of a so-called fifth wheel or some other towing device permanently mounted on a rear element of a framework of the tractor vehicle. During normal operation, particularly when a trailer is coupled to the tractor vehicle, the braking power of the tractor vehicle on the occasion of retardation is distributed between the front pair of wheels and the rear pair of wheels according to a predetermined fixed braking distribution which is usually such that a somewhat greater braking power is applied to the front pair of wheels than to the rear pair of wheels. Modern tractor vehicles are also often equipped with so-called ABS (Anti-lock braking system) brakes which, in brief, incorporate sensors designed to detect, and reduce the braking force applied to, any wheel which is tending to lock. These sensors are usually designed to detect the rotation speed of the wheels.
Such tractor vehicles usually have the engine situated in the vicinity of (or, in typical cases, above) the front axle. When such a tractor vehicle is driven forwards without a trailer, its center of gravity will consequently be relatively for forward and high up. In other words, in such situations the load on the front pair of wheels will be substantially greater than the load on the rear pair of wheels. Such a load distribution may give rise to problems due to powerful retardation causing more of the load to be shifted from the rear pair of wheels to the front pair of wheels, which means that the wheels i the rear pair will tend to rise off the ground and hence easily lock, which may impair the steering properties of the vehicle and cause great risk of the tractor vehicle getting into a skid. The problem described is particularly troublesome when the tractor vehicle is being driven downhill, which clearly entails the center of gravity being shifted forwards relative to the front axle of the vehicle.
A known way of solving this problem is to provide such tractor vehicles with a reducing valve, i.e. a constriction, on the brake line containing the medium which transmits the braking force to the brakes of the front wheels. This makes it possible to influence the pressure of the braking medium and hence reduce the braking force applied to the brakes of the front wheels, i.e. the predetermined braking distribution is altered by such a reducing valve so that a larger proportion of the braking power is distributed to the rear pair of wheels.
Such a reducing valve requires, however, a sensor to detect whether a trailer is coupled to the tractor unit, and a control unit to activate the reducing valve if no trailer is coupled to the tractor vehicle and deactivate the reducing valve if a trailer is coupled to the tractor vehicle. Moreover, reducing valves with different constrictions are required for different vehicles, depending on the weight and design of the vehicle. The reducing valve also entails an extra stage in tractor vehicle assembly. The cost of providing a tractor unit with such a reducing valve is therefore not insignificant.
JP09076891-A refers to an arrangement intended for a fork truck and designed to calculate permissible brake pressure by means of a processor. The calculation is based on a number of parameters such as load height, tilt angle, the vehicle""s angle of inclination, applied load, the vehicle""s acceleration, and incorporates determination of the center of gravity by means of a gravitation detector.
The object of the present invention is to ensure good braking action on a vehicle irrespective of its load state or of whether the vehicle is hauling a trailer or not. What is particularly aimed at is an arrangement which can at an early stage identify and avert an incipient forward tilting movement of the tractor vehicle.
This object is achieved with the arrangement indicated in the introduction, which is characterised by a control unit designed to control the braking action of the braking configurations of the front pair of wheels in response to the value of said parameter. Such an arrangement makes it possible to adapt the braking action on the front pair of wheels automatically to a critical state of the rear pair of wheels, particularly when the vehicle is not carrying a load and its rear end is of relatively low weight or the vehicle is being driven without a trailer. It is thus possible to prevent risk of the vehicle getting into a skid because the wheels of the rear pair lock or roll as a result of the powerful dynamic load shift forwards due to powerful retardation. This automatic adaptation of the braking action on the front pair of wheels may be simply accomplished by means of technology which is already incorporated in the tractor vehicles now on sale. Said parameter may with advantage be selected for the purpose in such a way as to indicate at least incipient locking of at least one of the wheels of the rear pair. Moreover, the control unit according to a preferred embodiment can be designed to reduce the braking action of the braking configurations of the front pair of wheels if the value of said parameter indicates at least incipient locking of at least one of the wheels of the rear pair.
According to one embodiment of the invention, said parameter pertains to the rotation of the wheels of the rear pair relative to the forward velocity of the vehicle. Such a state whereby the wheels of the rear pair rotate with a substantially lower rotation speed than that corresponding to the forward velocity of the vehicle is an indication that the rear wheels are tending to lock. Said means may therefore with advantage incorporate at least a first sensor designed to detect the rotation of one of the wheels of the rear pair. Said means may also incorporate at least a second sensor designed to detect the rotation of one of the front pair of wheels as a measure of the forward velocity of the vehicle. It should be noted here that said first and second sensors for detection of wheel rotation may also be used for detecting wheel retardation. Thus detection or substantially greater retardation on one of the rear wheels relative to an estimated vehicle retardation may be used as an indication of incipient wheel locking and may initiate reduction of the braking force applied to the front wheels. This technology is now used in many ABS systems for adjusting the braking force applied to a wheel found to be displaying a greater retardation. It should be noted that the invention may also use other means of identifying a need for reduced braking power.
According to another embodiment of the invention, said parameter relates to the load on the rear pair of wheels. A state whereby the load on the wheels of the rear pair is substantially below a normal value is also an indication that the rear wheels are tending to lose contact with the road and therefore lock. A load decrease on the rear wheels may thus be used for initiating a reduction in the braking action on the front wheels. Suitable load-detecting sensors may therefore, for example, be arranged to detect the pressure in pneumatic suspension devices of the tractor vehicle.
According to a further embodiment of the invention, the arrangement incorporates other means designed to detect the braking force applied by the braking configurations of the rear pair of wheels, and the control unit is designed to reduce the braking action of the braking configurations of the front pair of wheels according to the value of said parameter and that of said braking force applied. This makes is possible to cause the reduction in the braking action of the braking configurations of the front pair of wheels to decrease as said braking force applied increases. It is advantageous if the arrangement according to the invention can in this way detect and take into consideration whether the vehicle is in a state of powerful retardation. At the same time, the braking configurations of at least the rear pair of wheels may be activated by means of a braking medium supplied via a brake line, while said other means may with advantage incorporate at least a third sensor designed to detect the pressure of said braking medium.